The present invention relates generally to the field of purification systems. In particular, the invention relates to athermal desulfurization systems.
Most hydrocarbon or biomass derived fuels contain sulfur in excess of the level tolerable by fuel cell systems without loss in performance. This is particularly true of on-board fuel cell systems used as auxiliary power units. Most fuel cells typically give the best performance using pure hydrogen. Even a small percentage of sulfur (in the parts per million range) in the fuel gas can severely degrade the performance of the fuel cell. Solid oxide fuel cells, however, do not require pure hydrogen to operate. Solid oxide fuel cells are capable of operating on hydrocarbon fuels that produce carbon monoxide, which acts as a fuel to the electrodes in the fuel cells. While solid oxide fuel cells can run on fuel that contains hydrocarbons, the fuel must still be generally free of other contaminants, such as sulfur.
There are numerous mechanisms known in the art for removing sulfur from fuel. It is well known that certain high surface area solids can adsorb or chemisorb sulfur-containing molecules typically found in fuel including mercaptans, sulfides, thiophenes, thiophanes, and the like. Thus, one method currently being used to remove sulfur from fuel is to pass the sulfur-containing fuel through a sorbent bed. The sorbent bed adsorbs the sulfur from the fuel, resulting in a fuel that is either sulfur-free, or containing only a nominal amount of sulfur. However, sorbent beds can only adsorb a specified amount of sulfur before reaching a breakthrough point, at which time the sulfur begins to pass through the sorbent bed, making the sorbent bed less effective. Once the sorbent bed reaches the breakthrough point, it must be regenerated prior to reuse.
One of the methods currently being used to regenerate sorbent beds is to apply thermal energy to the sorbent bed in the presence of a flowing fluid in order to excite, desorb, and remove the sulfur-containing molecules from the sorbent bed. The sorbent bed is first heated to desorb the sulfur from the sorbent. After the sorbent bed has cooled down, it can be used to adsorb additional sulfur compounds. Thus, a heat exchanger is typically needed to regenerate the sorbent bed. Due to the need for high thermal energy and a bulky heat exchanger to regenerate the sorbent bed, it is often impractical to have a sorbent bed regeneration system on-board a moving vehicle, such as a jetliner or a truck. Additionally, the use of high thermal energy can often reduce the overall efficiency of the sorbent bed or significantly limit the life of the sorbent.